1. Technical Field
The present invention relates to a method used in a telecommunication network, a termination unit realizing the latter, and a telecommunication network including such a termination unit.
2. Discussion of Related Art
Such a termination unit is already known in the art, e.g. from the article xe2x80x9cCATV Return Path Characterization for Reliable Communicationsxe2x80x9d of Charles A. Eldering, Nageen Himayat and Floyd M. Gardner, published in the IEEE communications Magazine of August 1995 from page 62 to page 69. As described in the abstract of the article, there is a tremendous need for equipment to support bidirectional communications over a hybrid fiber coax HFC network shown on FIG. 1 of page 63. The HFC network is a tree branch network emanating from a central office and terminating at the subscriber residences in termination units which are called in the article coaxial termination units. Additional splitting of the downstream signal takes place in the residences in order to terminate in the subscriber terminals, e.g. personal computer, videophone, set-top box and TV.
It has to be remarked that the signal power attenuation of a transmission line between two points refers in this invention to the global attenuation of the transmission line taking into account the following parameters:
the attenuation of the transmission line proportional to the distance and the number of splittings along the line between the two points; and
the included amplifiers along the transmission line between the two points in the transmission direction.
A portion of the downstream signal power is provided to each subscriber terminal. Due to the attenuation along the network links, i.e. external signal power attenuation, and the attenuation of the growing in-house network in the residences, i.e. internal signal power attenuation, the portion of the power level of the downstream signal received in each of the subscriber terminals becomes low compared with the power level of the upstream signal generated by the subscriber terminal. This difference in power level makes the region of the internal network near the subscriber terminal very sensitive to cross-talk which influences the downstream signal.
This phenomenon is described in the book xe2x80x9cDigital, Analog, and Data Communication, Second Editionxe2x80x9d of William Sinnema with reference number ISBN 0-8359-1313-9, published in 1986 by Prentice-Hall, Inc., a division of Simon and Schuster, Englewood Cliffs, N.J. 07632, and more particularly on pages 115 to 117 thereof. On page 115 cross-talk is defined as induced signals into neighbouring pairs by either capacitive or inductive coupling because of the close proximity of adjacent wire pairs in a cable. As described on page 116 the crosstalk can be reduced by physically separating the downstream and upstream signals with a screened cable e.g. coaxial cable.
An object of the present invention is to provide a termination unit such as the above known ones but which is suited to further reduce the cross-talk in the region of the internal transmission line at the subscriber terminal.
According to a first aspect of the present invention, a method for use in a telecommunication network wherein a central station is coupled to a plurality of residences, one of said residences including a termination unit coupled to said central station via a first port, said residence including at least one subscriber terminal coupled to said termination unit via a second port, is characterized in that said method includes the steps of amplifying an upstream signal applied to said second port with an upstream gain value by means of an upstream signal amplifier included in said termination unit and generating an amplified upstream signal provided to said first port and that said upstream gain value is for compensating any of an upstream signal power attenuation of an internal transmission line coupled between said termination unit and any of said subscriber terminals, and a downstream signal power attenuation of said internal transmission line.
According to a second aspect of the present invention, a method for use in a telecommunication network wherein a central station is coupled to a plurality of residences, one of said residences including a termination unit coupled to said central station via a first port, said residence including at least one subscriber terminal coupled to said termination unit via a second port, is characterized in that said method includes the steps of amplifying a downstream signal applied to said first port with a downstream gain value by means of a downstream signal amplifier included in said termination unit and generating an amplified downstream-signal provided to said second port and that said downstream gain value is for compensating any of an upstream signal power attenuation of the internal transmission line coupled between said termination unit and any of said subscriber terminals, and a downstream signal power attenuation of said internal transmission line.
According to a third aspect of the present invention, a termination unit for inclusion in a telecommunication network wherein a central station is coupled to a plurality of residences, one of said residences including said termination unit coupled to said central station via a first port of said termination unit, said residence including at least one subscriber terminal coupled to said termination unit via a second port, is characterized in that said termination unit includes an upstream signal amplifier to amplify an upstream signal applied to said second port with an upstream gain value and to thereby generate an amplified upstream signal provided to said first port and that said downstream gain value compensates any of the upstream signal power attenuation of the internal transmission line coupled between said termination unit and any of said subscriber terminals, and the downstream signal power attenuation of said internal transmission line.
According to a fourth aspect of the present invention, a telecommunication network is characterized in that said telecommunication network includes at least one termination unit according to the third aspect of the present invention.
By including an upstream signal amplifier in the termination unit to amplify the upstream signal, according to the first, second and third aspects of the present invention, the required power level of the upstream signal generated by the subscriber terminal is allowed to be smaller.
Indeed, the subscriber terminal sends a less powerful upstream signal which is amplified in the termination unit in order to receive there the required powerbudget needed for a signal to be transmitted from the residence to the central station. The upstream gain value of the upstream signal amplifier is in fact a compensation for the upstream signal power attenuation of the internal transmission line coupled between the subscriber terminal and the termination unit.
In this way, at the region of the internal transmission line at the subscriber terminal, the difference in power level of the received downstream signal and of the transmitted upstream signal is smaller which makes this region less sensitive to cross-talk.
It also has to be remarked that in a totally symmetrical network the attenuation in upstream and downstream direction is identical, however e.g. the usable downstream and upstream frequency band for transmission in HFC networks are not the same. This makes for a situation in which the equipment to support bi-directional transmission e.g. the position of taps along the transmission line are optimized in one direction but not necessarily in the other direction. The following article, dealing with this problem can be mentioned as technical background: xe2x80x9cDesigning the Return System for Full Digital Servicesxe2x80x9d written by Dean A. Stoneback and William F. Beck and published in the proceedings manual of xe2x80x9c1996 Conference on Emerging Technologies, Jan. 8-10, 1996, San Francisco Hilton and Towersxe2x80x9d, by the Society of Cable Telecommunications Engineers, 140 Philips Road, Exton Pa. 19341-1318, (610) 363-6888, more particularly from pages 269-277. This is a reason why a distinction is made between upstream signal power attenuation and downstream signal power attenuation of the same transmission line. However, in order no to overload this introduction, the arguments are sometimes described for only one transmission direction or without specifying the transmission direction.
By including a signal amplifier in the termination unit to amplify the downstream signal, in further accord with the first, second and third aspects of the present invention, the power level of the received downstream signal is bigger at the subscriber terminal, which also makes the difference in power level of the received downstream signal and of the transmitted upstream signal smaller and the region becomes less sensitive to cross-talk.
The downstream gain value of the downstream signal amplifier is in fact a compensation for the downstream signal power attenuation of the internal transmission line coupled between the termination unit and the subscriber terminal.
A possible implementation of the first aspect of the present invention is that the gain of the amplifiers included in the termination unit is a function of:
the upstream signal power attenuation of the external transmission line coupled between the termination unit of the residence and the central station; or
the downstream signal power attenuation of the external transmission line.
An important advantage becomes clear with this feature. Indeed, the subscriber terminals are designed to be able to transmit an upstream signal to the central station with a certain power budget within a predetermined power level range in order to overcome the internal and external signal power attenuation along the transmission path from the subscriber terminal to the central station. The power level of an upstream signal received at the central station will become too low in the event that the total end-to-end signal power attenuation exceeds a maximum allowed signal power attenuation, i.e. each time the signal power attenuation of the external transmission line plus the signal power attenuation of the internal transmission line exceeds a maximum allowed signal power attenuation. This maximum allowed signal power attenuation is the signal power attenuation according to the upper limit of the predetermined power level range as described above.
Each time a new subscriber terminal is installed in the residence, the internal signal power attenuation should be determined again. Therefore a maximum expected internal signal power attenuation of an internal transmission line is defined and it can be made a requirement or rule that it may not be exceeded. This ensures that in order to respect the above requirement only the signal power attenuation of the external transmission line has to be taken into account. When the residence is located in the telecommunication network at a place where the external signal power attenuation is small, the above requirement is respected and there will be no receiving problem in the central station. However, when the residence is located in the telecommunication network at a place where the external signal power attenuation is big, the above requirement cannot be respected anymore.
By introducing the signal amplifier of the invention and by making the gain of the signal amplifier in the termination unit a function of the signal power attenuation of the external transmission line, the above requirement can again be respected.
Indeed, the extra power budget needed to overcome the signal power attenuation of the end-to-end transmission line is provided by the signal amplifier included in the termination unit.
It has to be remarked here that the decision to implement at the residence a termination unit according to the present invention and to activate the signal amplifier, depends on the quality of the end-to-end network. Indeed, once the external signal power attenuation exceeds the difference between the maximum allowed external signal power attenuation and the maximum gain value introduced by the signal amplifier, the power budget of the signal should be increased in order to have a power budget within the predetermined power level range of above. This is realized with the gain of the signal amplifier of the termination unit of the present invention.
Yet, a further characteristic feature of the first and second aspects of the present invention is that the upstream or downstream signal amplifier are controlled via control signals of a control unit. This feature corresponds to the fact that controllable gains of the amplifiers are an advantage for picking up changes in the level of the signal power attenuation of the external transmission line.
In a further implementation of the first aspect of the present invention, the signal power attenuation of the external transmission line is determined at set times whereby the value is provided to the control unit in order to determine the control signal of the downstream or upstream amplifier. In this way a remote automatic control system can be realized by using the method of the invention.
An application of the termination unit according to the second aspect of the present invention is a telecommunication network, according to the fourth aspect of the present invention, which includes at least one subscriber residence which includes a termination unit according to the third aspect of the present invention.